1. Field of the Invention
The present invention relates to a discharge-lamp lighting device for lighting and controlling a discharge lamp such as a metal halide lamp.
2. Description of the Related Art
Modern vehicles require increases in safety, adaptability to the environment and individuality. A vehicle headlight now needs to have an increase in the quantity of light, a reduction in size an improvement in running safety and a rise in design need. Conventional lamps used for the vehicle already encounter difficulties in meeting such a demand. Therefore, the adoption of a discharge lamp as a new light source to be mounted on the vehicle has been discussed.
FIG. 1 is a schematic view showing the structure of a 35 W type metal halide lamp corresponding to one kind of a conventional discharge lamp. The metal halide lamp 12 has a structure in which a silica tube 121 is sealed at its both ends and a luminescent or light-emitting tube 122 is situated in its center.
Reference numerals 123a and 123b indicate tungsten electrodes provided in the luminescent tube 122 and are electrically connected to external leads 125a and 125b respectively through molybdenum foils 124a and 124b respectively. The luminescent tube 122 has been charged, at its inside, with metal halogenides 126 produced by combining several kinds of metals such as sodium, scandium, etc. with iodine, a starter gas (such as xenon gas) 127 and mercury 128.
Such a discharge lamp is much different from the conventional light bulb in that the discharge lamp utilizes an arc created between electrodes as an illuminant wherein it is necessary to provide a lighting device for controlling the arc as compared with the conventional light bulb whose single filament is simply supplied with a voltage to emit light.
A description will now be made of a role to be performed by the lighting device through the manner of emitting light from the discharge lamp. The discharge lamp 12 first needs a high initial starting voltage of from several kV to ten and several kV. The lighting device generates the high voltage and applies it between the tungsten electrodes 123a and 123b of the discharge lamp.
Thus, an electric discharge is started between the tungsten electrodes 123a and 123b of the discharge lamp, so that a current flows between the tungsten electrodes 123a and 123b. Thereafter, the lighting device supplies the maximum rated power or current of the discharge lamp 12 to the discharge lamp 12 to thereby increase the quantity of light emitted by or from the discharge lamp 12 as soon as possible.
At this time, the current, which has flowed in the discharge lamp 12, activates the starter gas 127 charged into the luminescent tube 122 to thereby start an arc discharge based on the starter gas 127.
At this time, the voltage applied to the discharge lamp 12 rises from about 20 V. Further, the lighting device adjusts power to be input or supplied to the discharge lamp 12 to gradually decrease the power in accordance with this voltage to thereby control or adjust the quantity of light emitted from the discharge lamp 12 in an overload state.
When the power to be supplied to the discharge lamp 12 is controlled, the temperature at the inside of the discharge lamp 12 rapidly rises to evaporate the mercury 128, with the result that an arc discharge based on mercury gas is then started. Since the temperature at the center of the mercury arc reaches about 4500K (Kelvin) and the inside of the luminescent tube 122 is brought to a higher temperature and a higher pressure, the metal halogenides 126 start evaporating and are separated into a metal ion and a halogen ion within the arc. As a result, the metal ion emits light at a spectrum peculiar to the metal.
After the vaporization of substantially all the metal halogenides 126, arc light reaches a final form and output and the voltage of the discharge lamp 12 is saturated so as to reach a stable voltage (hereinafter called a "stationary lamp voltage"). At this time, the lighting device fixes the power to be supplied to the discharge lamp 12 to the rated power to thereby emit stable light free of any flicker from the discharge lamp 12.
It is thus necessary for the lighting device to actively control the power to be supplied to the discharge lamp 12 based on the lamp voltage in order to cause light to rapidly rise and stabilize. A method of effecting such power control has been described in a standard of EUREKA PROJECT 273 VEDILIS (hereinafter called simply "VEDILIS") shown in FIG. 2 as one example.
FIG. 2 shows a lamp current control characteristic indicative of lamp current (i.e., power) flowing into a discharge lamp vs. desired lamp voltages applied to the discharge lamp. According to the lamp current control characteristic based on the VEDILIS, lighting rise control is effected within a range in which the maximum rated power and the maximum rated current of the discharge lamp are satisfied, and the discharge lamp is finally lighted and controlled at the rated power.
First of all, a current less than or equal to the maximum rated current is caused to flow in the discharge lamp in the lamp-voltage range of 0 V to a voltage of 28.8 V determined from the maximum rated power/maximum rated current of the discharge lamp. During this period, a linear characteristic appears on the lamp current control characteristic. A current, which provides the maximum rated power with respect to the lamp voltage, is allowed to flow in the discharge lamp in the lamp-voltage range of 28.8 V to a desired voltage (40 V in this example). During this period, a curve characteristic appears on the same lamp current control characteristic.
Next, a linear characteristic appears in the lamp-voltage range of 40 V to the minimum rated voltage 65 V of the discharge lamp. When the lamp-voltage range exceeds such a lamp-voltage range, a lamp current, which lights the discharge lamp at the upper limit of the rated power of 38 W with respect to the lamp voltage, is allowed to flow in the discharge lamp. During this period, a curve characteristic appears on the same lamp current control characteristic.
To sum up, the linear characteristic, the curve characteristic, the linear characteristic and the curve characteristic appear on the lamp current control characteristic based on the VEDILIS in this way until the lamp voltage is saturated and stabilized.
FIG. 3 shows a lamp current control characteristic obtained by a lighting device which has been disclosed in Japanese Patent Application Laid-Open Publication No. 4-141988. The lighting device effects power control along a straight line gc and a straight line gb placed at an angle of h during a period (transition region Ab) in which a region changes from a light-emission exciting region Aa at which the maximum rated current is caused to flow in a discharge lamp in accordance with a lamp current control characteristic indicated by a straight line ga to a constant power region B at which constant power control is effected in accordance with a lamp current control characteristic indicated by the straight line gc. The lighting device effects power rate-of-change reducing control along a curve h which is smooth in the vicinity of a point at which the straight line ga and the straight line gb intersect. Symbol PQ in FIG. 3 indicates a constant power curve.
When the conventional discharge-lamp lighting device is constructed as described above, the luminous efficiency of the discharge lamp at a desired lamp voltage which varies due to variations in its manufacture and its secular change, has not been taken into consideration. Further, only control based on a fixed lamp current control characteristic can be effected. In almost all cases, because the fixed lamp current control characteristic is directed toward its minimum rated voltage, there is developed a power shortage in the vicinity of a point where the lamp voltage reaches a stationary lamp voltage. As a result, a large undershoot occurs in light output produced from the discharge lamp.